The invention is directed to a liquid crystal display unit of the type having a matrix of pixels which can be individually controlled.
A liquid crystal is an organic fluid which, despite being a fluid, has molecules which spontaneously assume an ordered configuration. The configuration can be altered by applying an electric field, and the resulting change in the configuration of the molecules can be used to control light passing through the liquid crystal.
A class of liquid crystals known as nematic liquid crystals is frequently used for making electrical displays. The liquid crystal is sandwiched between two polarizers having transparent electrodes made, for example, from metal oxide film. The polarization axis of light passing through the first polarizer is rotated by the liquid crystal if no voltage is applied between the electrodes, and the degree of rotation decreases or disappears entirely when a voltage is applied. As a result, light passing through the first polarizer and the liquid crystal may or may not pass through the second polarizer depending upon the applied voltage and hence the polarization axis of the light when it reaches the second polarizer. The electrodes are normally patterned to provide individual regions which act as light valves. The individual regions may, for example, be configured to provide a seven-segment numerical display of the type frequently found in electrical watches. The voltages applied to various segments determine which segments are visible and thus which number appears.
The patterned electrode regions may also be disposed in a matrix which provides picture elements or pixels that can be individually controlled. Performance of the display panel can be improved by providing an amorphous MOS transistor for each pixel. Such display panels, with a transistor for each pixel, will be called "active-matrix" display panels in this application. An active matrix display panel is described, for example, in J. Wilson et al., "Optoelectronics, An Introduction" (1989) pages 144-152, or Donald G. Fink et al., "Electronics Engineers' Handbook," Third Edition (1989), pages 20-102 to 20-103. In contrast to active matrix display panels, LCD display panels without a transistor for each pixel will be called "passive-matrix" display panels in this application. The term "matrix-type" display panel, or simply "matrix" display panel, will be used generically to refer to either an active or a passive matrix panel.
Matrix-type liquid crystal display panels have been employed as television screens. In low-power designs, ambient light is reflected onto the back of the LCD panel. However, back-lighting with a white fluorescent lamp is frequently used. Furthermore, both modes of illumination may be provided so that the user can select the mode most appropriate to the viewing conditions. For example, the Seiko model "TFT" Pocket Color Television employs a back-light assembly which can be turned ON or OFF. The back-light assembly can be pivoted to a position directly behind the LCD panel when the back-lit mode is used, or it can be pivoted outward and used as a reflector of ambient light.
A matrix-type LCD panel basically provides a black-and-white display. However, it is known to convert this to a color display by attaching color filters to the LCD panel. One pixel is exposed through a red filter, an adjacent pixel is exposed through a green filter, and a third pixel adjacent the first two is exposed through a blue filter. As a result, the three adjacent mono-color pixels effectively provide a region in which the color can be controlled, this region encompassing the three pixels.
One difficulty with the conventional color LCD panel with mono-colored pixels, achieved with colored filters attached to the panel, is that resolution suffers. Each color-controllable region encompasses three pixels, and since the ultimate image is composed of such color-controllable regions the image is coarser than one would expect from a comparable black-and-white LCD panel having the same number of pixels.